1. Field of the Invention
The present invention relates to a toner for image formation, a method for producing a toner for image formation, a toner container, a two-component developer, a process cartridge, and an image forming method.
2. Description of the Related Art
In electrophotographic image forming apparatus, electrostatic recording apparatus, etc., electric or magnetic latent images are visualized by a toner. In electrophotography, for example, a toner image is produced by forming an electrostatic image (latent image) on a photoconductor and developing the image by a toner. The toner image is generally transferred onto a transfer material (e.g., paper) and fixed thereon by heating or the like. The toner used for development of latent electrostatic images is generally composed of colored particles prepared by adding a colorant, a charge control agent and additional additives in a binder resin. The toner production methods are broadly classified into pulverization methods and suspension polymerization methods. In the pulverization method, a colorant, a charge control agent, an offset inhibitor and other agents are melt-kneaded with a thermoplastic resin and uniformly dispersed therein, and the resultant composition is pulverized and classified to produce a toner.
The pulverization method can produce a toner with excellent characteristics at a certain degree, but the latitude is limited in the selection of toner materials. For example, the toner composition prepared by melt-kneading of toner materials needs to be capable of being pulverized and classified with devices which can be operated economically. This requirement necessitates that the melt-kneaded composition be sufficiently fragile. For this reason, when the toner composition is pulverized into particles, it becomes likely that a broad particle size distribution is produced, and therefore, in an attempt to produce a high-resolution copy image with many levels of gray, it is necessary to remove, for example, fine particles with a diameter of 5 μm or less, particularly 3 μm or less, and coarse particles with a diameter of 20 μm or more by classification. This leads to very low toner yield. By the pulverization method, it is difficult to uniformly disperse such agents as a colorant and a charge control agent in thermoplastic resin. Moreover, by the pulverization method, the colorant component added in the toner is undesirably exposed to the toner surface and thereby the charge distribution becomes uneven over the toner surface, leading to a broader toner charge distribution and poor developing characteristics. Thus, the current situation is that kneading/pulverization methods cannot fully satisfy the requirements of producing high-performance toner owing to these problems.
In recent years, toner production methods using suspension polymerization have been suggested and put into practice as methods that can overcome the above-mentioned problems pertinent in pulverization methods. Production of a toner for developing a latent electrostatic image by polymerization is a conventionally known technology; for example, current toner particles are produced by suspension polymerization. Toner particles produced by suspension polymerization, however, have substantially spherical shape and thus are hard to be removed. In the case of low-image coverage development and transferring, the amount of residual toner particles is small and thus cleaning failure is not significant. However, cleaning failure becomes significant in the case of development and transferring of a high image coverage object, such as a picture image. Moreover, toner particles that have been used for development but remained untransferred due to paper feed failure or the like reside on the photoconductor as residual toner particles and cause background smear when accumulated.
Such residual toner particles smear on, for example, a charging roller, which is brought into contact with and charges the photoconductor, preventing it from exerting its original charging ability. Moreover, since the toner is produced at the same time when a resin is produced by suspension polymerization, it is often the case that toner materials used for conventionally known toners cannot be used in suspension polymerization. Even when polymerization is successfully effected using conventionally known materials, in some cases, the particle size cannot be fully controlled due to influences of resin and additives such as colorant. Thus, one of the problems associated with suspension polymerization is its limited latitude in the selection of materials, with the major problem being the fact that polyester resins, which offer excellent toner fixing property and coloring property when employed in conventionally known kneading/pulverization methods, cannot be generally employed and, therefore, this method cannot be used in view of growing demands for smaller, faster color printers. To overcome the problem pertinent in suspension polymerization, for example, Japanese Patent (JP-B) No. 2537503 discloses a method of producing randomly shaped toner particles by aggregating fine resin particles produced by emulsion polymerization.
In the toner particles produced by emulsion polymerization, however, a large number of surfactant components remain not only on the toner surface, but inside the toner even after washing process, leading to poor toner charge stability and broader charge amount distribution, which in turn causes background smears on the obtained image. In addition, the remained surfactant smears on the photoconductor, a charging roller, a developing roller, and other members, preventing them from exerting their original charging ability. Even in the case of emulsion polymerization where the colorant components are hardly exposed to the toner surface, it is difficult to uniformly disperse the colorant in the toner since the colorant components are easily aggregated together. Because the manner in which colorant exists differs between individual toner particles, there are variations in charge amount among toner particles and thus toner stability over a long period decreases. In addition, in the case of color printing, slight reductions in developing ability and transfer ability leads to poor color balance and poor gray scale. Furthermore, since the colorant in the toner particles is generally hydrophilic and is not compatible with resin, diffused reflection of transmitted light occurs at the interface of surfactant and resin components, reducing the transparency of OHP sheets and the like when printed. Namely, when the colorant is not sufficiently dispersed in the toner, the transparency of the printed OHP sheet reduces.
Japanese Patent Application Laid-Open (JP-A) No. 2001-66827 discloses a toner produced by the method including the steps of dissolving or dispersing in a first organic solvent capable of dissolving a binder resin, a pigment dispersant and a pigment that has been surface-treated with a fatty acid, so as to prepare a pigment dispersion solution, of mixing a binder resin with the pigment dispersion solution in a second organic solvent capable of dissolving binder resin, so as to prepare an oil component, of suspending the oil component in an aqueous medium to form microdroplets of the oil component, and of removing the solvent from the suspension. However, fatty acids contain no amino groups that control toner charging ability.
JP-B No. 3661422 discloses a toner produced by using a polymer dispersant as a pigment dispersant. This disclosure specifies the acid value and amine value of the polymer dispersant so as to provide a toner that offers excellent offset resistance, charging ability, storage stability, color developing ability, and OHP transparency. However, storage stability, particularly resistance to “blocking” that occurs during toner delivery, are insufficient. In this disclosure, a synergist, or a pigment derivative, is added as a pigment dispersant. This synergist can enhance pigment dispersibility by introducing polar groups into the pigment so as to increase its interactions with the pigment dispersant. However, when the synergist is used in the production of so-called chemical toner, where toner is prepared in an aqueous system, it results in unwanted migration of pigment components toward toner surface or into the aqueous phase during toner production. The causes of these phenomena still remain elusive. In general, synergists are considered to adsorb to surfaces of pigment components, where they introduce polar groups into the pigment so as to increase its interactions with a pigment dispersant. The polar groups of synergists are considered to be generally hydrophilic, suggesting that migration of pigment component toward toner surface or into the aqueous phase occurs during toner production. These phenomena lead to reduced coloring ability and reduced color saturation, and/or poor fixing characteristics, and furthermore, leads to pigment smear on other members.